1. Field of the Invention
The invention relates in general to tachogenerators, and more specifically to friction driven tachogenerators.
2. Description of the Prior Art
U.S. Pat. No. 4,030,570, which is assigned to the same assignee as the present application, discloses a traction elevator system and the importance of developing motor velocity, motor acceleration, and stabilizing feedback signals for the motor control loop which are free of electrical noise. The accelerator signal is used to provide system damping and to significantly reduce oscillations due to the natural, lightly damped, resonance of the mechanical system of discrete masses coupled together with "springy" cables. This is especially important when a solid state dual bridge converter drive is used, because of its fast response and ability to follow noise in a signal. Noise in the signals can cause the mechanical system of the traction elevator to be perturbed and generate an annoying vertical vibration of the elevator car. This patent also discloses that electrical noise can be reduced in the signals by generating them with a high quality tachogenerator having a ripple of less than 2% peak-to-peak, and by friction or rim driving the tachogenerator, eliminating noise due to belts or gears.
With a friction drive, it is important that no slippage occur between the friction wheel and the surface of the movable element which rotates the wheel. To increase the force of the friction wheel against the movable element, special tachogenerators are used which have a larger diameter shaft and heavier duty bearings, than conventional tachogenerators. This special tachogenerator is hinged mounted, with a biasing force being applied to the tachogenerator which in turn biases the friction wheel against the driving surface through the bearings and shaft of the tachogenerator. Recent experience has shown that friction in the hinge can interfere with the ability of the tachogenerator to follow the higher frequency deviations in the velocity while following the steady-state and lower frequencies very accurately. This inability of the tachogenerator to sense the higher frequency velocity components inhibits the desired increase in damping and the elevator ride smoothness is significantly impaired.
It would be desirable to eliminate this frequent maintenance, while providing a friction driven tachogenerator system in which the complete biasing force will always be applied to the friction wheel. In other words, it would be desirable to eliminate friction points which can reduce the percentage of the biasing force which actually ends up at the interface between the friction wheel and the movable driving element. It would also be desirable to accomplish these objectives while reducing the overhung load on the shaft and bearings to the point where standard shaft and bearing arrangements may be used, substantially reducing the cost of the tachogenerator, and also reducing its physical size.